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"dead" or "hot" pixels?

 
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cube-tube



Joined: 10 Oct 2017
Posts: 105
Location: Durham, NC

PostPosted: Mon Jun 17, 2019 3:30 pm    Post subject: "dead" or "hot" pixels? Reply with quote

To me these multicolored "dust trails" look like dead pixels. Has anyone else experienced these? Could it be some other kind of sensor noise? Can they be minimized with a shorter exposer?

I'm shooting at base ISO on a Canon 6D, ~8 second exposure, with a single flash at ~7 second mark.

Thanks

100% crop:

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rjlittlefield
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Joined: 01 Aug 2006
Posts: 19980
Location: Richland, Washington State, USA

PostPosted: Mon Jun 17, 2019 5:07 pm    Post subject: Reply with quote

I see you have a lot of these and they vary in intensity. That combination indicates these are warm/hot pixels, not dead. You have more than I usually see, but yes, they're common with long exposures.

Ultimately the problem of warm/hot pixels is just that some pixels leak charge faster than others and after a while those pixels have too many "bad" electrons caused by leakage, versus "good" electrons caused by photon capture. Warm/hot pixels get worse with long exposures and higher temperatures because those give more opportunity for leakage charges to accumulate.

Most people see warm/hot pixels when using multi-second exposures with continuous illumination. In that case the cure is to add light so as to shorten the exposure time. Adding light to reduce ISO at same shutter speed will also help. It also helps to avoid Live View mode during actual shooting, because continuous use of the sensor during Live View causes the sensor to get warmer.

Your situation is a little unusual because you're already running at base ISO and flash, so the only good place to attack is the long delay in blackness. I suggest to start by just shortening that to see if you can get rid of the warm pixels without getting motion blur caused by shutter action. Remember to use mirror lockup. Adding more flashes and/or moving the flash head closer to subject can also help, by allowing the flashes to run on lower power and thus produce shorter pulses that do a better job of freezing motion. Flash at say 1/16 power, giving effective exposure times of 1/5000-1/10,000 second, is pretty effective at freezing shutter vibration.

If you are currently using Live View and controlling the flash with external trigger, then I suggest to stop doing that because of the sensor heating issue. Instead, let the sensor run cool without Live View and let the camera trigger the flash on second curtain.

--Rik
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Lou Jost



Joined: 04 Sep 2015
Posts: 3644
Location: Ecuador

PostPosted: Mon Jun 17, 2019 5:27 pm    Post subject: Reply with quote

I've seen people on the internet modify their cameras with Peltier coolers. Has anyone tried that? This would not only reduce hot pixels but also reduce noise generally.
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zed



Joined: 07 Feb 2019
Posts: 25
Location: Houston, TX

PostPosted: Tue Jun 18, 2019 6:28 am    Post subject: Reply with quote

If the flash is your primary light source then as Rik states - increasing shutter speed and using mirror lockup instead of leaving the shutter open will help. 8 second exposures with flash is too long. Ideally you want to image as close to your camera's flash sync speed as you can - and if the flash is the only light source you should see no difference between an exposure taken at 1/250 vs 1 second.

Also - just a note on the mirror lockup - you want to leave a second or 2 in Zerene between the first and second curtains to allow for shutter dampening. Depending on how isolated your system is - you may need longer or shorter values here. I use a 2 second gap here.

Alternatively this might be a good point to practice your cloning/healing skills in PS. You'll want to correct for the dust particles in the image anyway - so adding hot pixel corrections doesn't add too much to the processing time.
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mawyatt



Joined: 22 Aug 2013
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Location: Clearwater

PostPosted: Tue Jun 18, 2019 6:49 am    Post subject: Reply with quote

All,

What using a "Dark Frame" captured with the same setup conditions and exposure time with the optical path blocked (no light), or just without any ambient light.

Subtract the "Dark Frame" from each captured image before stacking. This should remove or dramatically reduce the "Hot Pixels" and also any pattern noise.

Would be a nice feature in Stacking Software, where a Dark Frame is identified, then subtracted before stacking each individual image, or done prior to the stacking routine.

Best,
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Lou Jost



Joined: 04 Sep 2015
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Location: Ecuador

PostPosted: Tue Jun 18, 2019 7:03 am    Post subject: Reply with quote

Mike, that would be a great addition to stacking software!!!
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mawyatt



Joined: 22 Aug 2013
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PostPosted: Tue Jun 18, 2019 7:20 am    Post subject: Reply with quote

Nikon can remap hot pixels by using the "Clean Sensor" feature twice in succession evidently. See this thread, later part.

https://www.dpreview.com/forums/thread/3337623?page=2

Awhile back my D800 had a couple hot pixels that just disappeared after actually cleaning the sensor. I think I had issued the "Clean Sensor" a few times (without knowing this would remap the hot pixels) before doing an actual chemical cleaning.

Best,
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cube-tube



Joined: 10 Oct 2017
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Location: Durham, NC

PostPosted: Tue Jun 18, 2019 7:56 am    Post subject: Reply with quote

Thanks all.

I'll try decreasing the blackout time in my exposure.

I'm running a single speedlight (evolv 200) at 1/128 power, but it overheats if I make more than one exposure every 10 seconds, which is how I arrived at 10 seconds of darkness/no motion, and 2 seconds to close and open the shutter and advance the stage.

I'll also try cleaning my sensor.

I really can't imagine adding a Peltier cooler to this camera. I've seen people do it with CCDs for astronomy, though.
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mawyatt



Joined: 22 Aug 2013
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Location: Clearwater

PostPosted: Tue Jun 18, 2019 9:20 am    Post subject: Reply with quote

cube-tube wrote:
Thanks all.

I'll try decreasing the blackout time in my exposure.

I'm running a single speedlight (evolv 200) at 1/128 power, but it overheats if I make more than one exposure every 10 seconds, which is how I arrived at 10 seconds of darkness/no motion, and 2 seconds to close and open the shutter and advance the stage.
.


The Adorama Studio Strobe 300 (Godox) can be repeatedly used with less than 2 second intervals at low power settings. This is what I normally use for massive S&S sessions. For ~$120 they are a great value IMO.

Best,
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